It is often desired to analyze laboratory samples under fixed conditions of temperature and pressure in scientific experiments. In a typical scientific laboratory environment, temperature fluctuations occur at random. Various devices such as temperature controlled water baths, incubators and refrigeration rooms have been employed in an attempt to combat these fluctuations; however, these devices are expensive and cannot maintain extremely low or extremely high temperatures.
Analyses which must be performed at cryogenic temperatures are often conducted within a bath of liquid nitrogen. Liquid nitrogen boils at room temperature making frequent replacement of the evaporated liquid necessary. In lengthy experiments this need for constant replacement is often inconvenient.
An additional problem encountered in the analysis of samples under cryogenic conditions is that, if a sampe being analyzed is contained within a vessel, and only a portion of the vessel is immersed in the liquid, a temperature gradient is produced within the vessel above the sample. The bottom of the vessel, in contact with the liquid, is at the cryogenic temperature, but the gases within the non-immersed portion of the vessel become progressively warmer as the top of the vessel is approached, even if the vessel is a closed container. The gradient exists because the air outside the vessel ranges from a very low temperature, near the evaporating cryogenic liquid, to ambient temperature at a particular height above the cryogenic liquid. These gradations in temperature affect primarily the portion of the vessel not submerged in the liquid. The air outside the vessel, immediately above the surface of the cold liquid, is cold because it contains cold gaseous molecules formed from the evaporating liquid. These cold gaseous molecules are warmed by adjacent air molecules and become dispersed in accordance with the second law of thermodynamics, so that, the farther a gas molecule rises from the surface of the liquid, the warmer it becomes. The heat from the warmed gas and air molecules outside the vessel is conducted through the walls of the vessel to the gases contained within the vessel. Therefore, the temperature of the gases within the vessel is not constant and the continuing evaporation of the cryogenic liquid increases the length of the temperature gradient.
The need for a static environment is critical during the measurement of the surface area and pore volume of a solid sample. The measurement of surface area and pore volume is often determined by instruments which obtain points for a BET curve, such as shown in U.S. Pat. No. 3,850,040. Small pressure changes in the gases within a vessel surrounding a sample at constant temperature are measured as part of the determination of surface area and pore volume values. An accurate reading of pressure can only be made under conditions of non-fluctuating temperature, most frequently a stable cryogenic temperature, because uncontrolled changes in temperature create uncontrolled changes in pressure. This can lead to errors in the measured amount of gas adsorbed by the sample.
Several prior devices have been developed to maintain gases at a constant temperature within a vessel. In one device, the cryogenic liquid is contained within a Dewar flask, and the entire flask is raised at the same rate that evaporation occurs, thereby keeping the level of the liquid at the same height with respect to the vessel. The major disadvantage to this device is that, as the Dewar flask is raised, it surrounds more and more of the previously exposed portion of the vessel and traps the cold gas molecules as they evaporate from the cryogenic liquid, thereby shifting the gradient and creating uncontrolled temperature and pressure changes within the vessel.
Another device, described in U.S. Pat. No. 3,850,040, transfers fresh liquid to the Dewar flask at the same rate that evaporation occurs, so that the height of the liquid remains at a constant level. In this device the temperature gradient does not shift. The major disadvantage to this device is that, in humid climates, ice can accumulate within the values and seals on the apparatus used to pump the liquid from the reservoir to the Dewar flask and can cause the device to fail.
Thus, there is a need for a simple device that maintains an evaporating liquid at a constant height in order to keep the gases within a vessel immersed in the liquid at a constant temperature.